The use of polyvinyl alcohol as a separator material in alkaline batteries is well known. A standard electrolyte for alkaline batteries is concentrated aqueous potassium hydroxide. Although polyvinyl alcohol is soluble in water, it is not readily dissolved in such a concentrated potassium hydroxide solution, and thus, films of this material can be used in such electrolytes. In general, polyvinyl alcohol films have been utilized as battery separators because of their high conductivity in alkaline electrolytes and their ease of fabrication from aqueous solution.
Efforts have been made to improve the mechanical properties and chemical stability of polyvinyl alcohol by synthesizing numerous derivatives of the polymer. However, the number of processes for the in-situ reaction of prefabricated polyvinyl alcohol structures, such as battery separators, is limited. The in-situ acetalization of polyvinyl alcohol separators has been achieved through treatment of the separators with acid solutions of aldehydes, such as an aqueous formaldehyde solution containing a small amount of sulfuric acid. The aldehyde acetalates the 1,3 diol units present in the polymer thereby creating tough, water insoluble derivatives. In many of the aldehyde treatments, the extent of acetalization is difficult to control. Extensive acetalization of polyvinyl alcohol leads to poor conductivity in alkaline electrolyte, which is probably due to the decreased number of hydrophilic alcohol groups resulting from the using up of such groups by the acetalization reaction.
It is an object of the present invention to provide a method of making cross-linked polyvinyl alcohol and other water soluble polymeric sheet structures in which the extent of cross-linking is easily controlled. It is a further object to provide cross-linked polyvinyl alcohol structures having low electrical resistivity suitable for use as a separator or electrode envelope in an alkaline battery.